Heretofore, in the field of optical communications and in the sub field of Identification Friend or Foe various attempts have been made to develop an omni-directional transponder that will remain covert until interrogated with the proper optical beam and only then responds with the proper message. These attempts have included modulating devices, such as polarizers and acousto-optic modulators. However, these devices are functionally dependent upon the interrogating wave length, polarization, and angle of incidence. Under some conditions these devices will return optical energy in the "off mode" thus destroying their covert characteristics.
To overcome this deficiency a mechanical shutter was placed in front of the retro modulator so that the modulator could maintain its covertness, or fail to return optical energy in the presence of a unauthorized interrogation. However these shutters were bulky and relatively slow in response. As a result, attempts have been made to use retro reflective characteristics of lenses to modulate the signal at the lens focal plane. This attempt has been accomplished by moving the reflecting surface at the focal plane or by modulating the reflectivity at the focal plane by various modulation techniques. Dr. Buser, Director of the Army's Center for Night Vision and Electro Optics notes in his U.S. Pat. No. 4,361,911 entitled "Laser Retroreflector System for Identification Friend or Foe": "At the present time there is no known wide field-of-view laser retroreflector which can be interrogated successfully and yet remain covert" (Col. 1, lines 21-23). In this patent Buser teaches a lens system with an acousto-optic modulator at the focal plane to return the signal. However, these lens systems are bulky and have a limited field of view when compared to a corner cube.
There have been several designs using Frustrated Total Internal Reflection (FTIR) to accomplish switching or modulation of a beam of light. In almost all cases these systems begin with an air gap which produces total internal reflection, and then rapidly drives the material to less than one tenth wave length spacing to produce frustrated total internal reflection. These systems are typified by U.S. Pat. Nos. 4,249,814; 3,649,105; 3,559,101; 3,376,092; 3,338,656; 2,997,922; and 2,565,514. U.S. Pat. No. 3,514,183 teaches the use of a device in which the edges of a transparent member are attached to a prism and the center of the secondary glass plate is pulled away from the prism. In all of these systems there is a problem in overcoming stiction and damage to the glass. These systems experience two specific problems. To achieve contact closure in a short time requires that the two surfaces be driven together with great force. The high rate of deceleration that the surfaces experience when they come in contact with each other causes cold welding and fracturing thus limiting the useful life of the device. In addition when the two surfaces are pulled apart with a uniform force across their interface or from the center there is a vacuum that is formed between the two surfaces, and a great deal of force is required to overcome the stiction.
Most of the systems have been designed to operate at a single wave length and a single angle of incidence, near the critical angle. Many of these designs acknowledge that the reflection does not go to zero. To overcome the problem of stiction and damage U.S. Pat. No. 4,165,155 teaches a device with zero reflectivity when operating at a spacing of 1 wave length. However, this device will work for only one wavelength and one angle of incidence, and thus would not be covert for a wide range of angles and wavelengths.
Previous systems have focused the design on angles near the critical angle and have not concerned themselves with the residual reflections for separations between 0 and 1/10 .lambda.. As will be seen in FIG. 12 at the higher angles of incidence the residual reflection at spacings on the order of 1/10 .lambda. are significant.
Accordingly, a need has arisen for a Frustrated Total Internal Reflection Modulator that a) overcomes the problem of stiction; b) is in the off mode when not activated; c) will reduce the minimum reflection to zero over a wide range of incident angles and wave lengths; and d) is covert in the off mode over a wide range of angle and wave lengths.